1. Field of the Invention
This invention relates to a two-pole overcurrent protection device for connection into a current-carrying conductor. In particular the two-pole overcurrent protection device is of the kind comprising a normally conducting thyristor provided with turn-off means and having, in the following order, a first emitter layer, a first base layer, a second base layer, and a second emitter layer, and an MOS transistor integrated with the thyristor, connected in parallel with the PN-junction between said first emitter layer and said first base layer and arranged, when the voltage across the thyristor exceeds a predetermined value, to short-circuit the said PN-junction for turning off the thyristor.
2. Description of the Prior Art
A two-pole overcurrent protection device of the kind referred to has previously been disclosed in U.S. Pat. No. 4,331,884 (issued on May 25, 1982 to Per Svedberg and assigned to the assignees of this application). In this known overcurrent protection device, zener diodes are arranged to protect control means of the field effect transistor parts from impermissibly high voltages. These zener diodes are made in the same integrated circuit as the other parts of the overcurrent protection device. However it has proved that when one of the two thyristor portions of the overcurrent protection device is current-carrying, the charge carriers then generated give rise to large leakage currents in the zener diodes, which has an adverse effect on the functioning of the known overcurrent protection device.